1) Field of the Invention
The present invention relates to a curtain coating apparatus and curtain coating process for producing coated products of a good quality such as printing sheets, pressure-sensitive recording sheets, heat-sensitive recording sheets, photographic sheets, printing process materials, etc. by eliminating entrained air about a continuously moving web such as paper sheets, paper sheets to be coated, films etc. at a high speed coating, thereby stabilizing the curtain film.
2) Related Art Statement
Curtain coating process is a process, which comprises forming a free falling liquid curtain film of at least one of coating solutions, and allowing the liquid curtain film to contact with a material to be coated (web), thereby forming a film on the web to be coated, and has been used for a long time as a process for surface-coating steel plates.
The curtain coating process has so far suffered from uncoated regions or coating unevenness during the formation of films on the web. One reason is a poor affinity of the web surface to the coating solution. In that case, such a problem has been solved by improving the affinity of the web to the coating solution by a physical or chemical means.
Another reason is a problem of entrained air around the web surface. The entrained air is generated on the high speed-moving web surface, and the coating solution is applied to the web surface without eliminating the entrained air from the web surface, thereby forming uncoated regions on the coated web surface or the entrained air turns to a turbulent air flow near the liquid curtain film to vibrate the liquid curtain film, thereby generating coating unevenness on the coated web surface. Such problems will be serious particularly at a higher coating speed, and thus it is the main reason why the coating speed cannot be increased in the curtain coating process.
In a slide hopper type curtain coating apparatus as a typical curtain coating apparatus for coating a photographic photo-sensitive material, the heel part on the coating line is attracted toward the web upstream side by evacuation at the coating point (contact point between the web and the coating solution), thereby stabilizing the coating, and also solvent vapor are generated at the same time as the evacuation or the entrained air is replaced with a gas easily soluble in the coating solution (e.g. carbon dioxide gas) as further supplementation means.
In an extrusion type curtain coating apparatus, on the other hand, it is possible to conduct coating in a high speed region than in the slide hopper type curtain coating apparatus, because the falling momentum of curtain film can break the entrained air zone. However, with increasing coating speed or to make the generation of product quality defects zero, many attempts have been made to eliminate the entrained air itself. Basically, it has been proposed to provide an air shield plate for shielding the curtain film from the entrained air at a position upstream of the curtain film. Since then, still further attempts have been made. A typical one is a method for withdrawing the entrained air at a position upstream of the curtain film by suction, but the entrained air newly generated in the high speed region cannot be fully shut off from the curtain film by such an entrained air-withdrawing method.
A method of contacting a web with a shield at a position by a few ten millimeters upstream of the curtain film (which will be hereinafter referred to as xe2x80x9ccontact typexe2x80x9d) has been also proposed as an effective means for fully shielding the curtain film from the entrained air. The contact type is an ultimate method for shielding the curtain film from the entrained air, because it physically shields the curtain film from the entrained air by means of a solid shield, and it has been indeed difficult and not efficient to shield the curtain film from the entrained air by other methods better than by the contact type.
Contact type curtain coating apparatus will be described in detail below, referring to FIG. 5, which is a schematic view of a conventional curtain coating apparatus, where shield member 21 is provided at a position upstream of and near coater head 1. Web contact member 22 is provided at the lower end of shield member 21 and is brought into contact with web 10 during the movement of web 10. Air entrained by high speed movement of web 10 is shut off from curtain film 6 by web contact member 22, thereby giving no influence on curtain film 6 at the downstream position.
However, in the contact type the shield is in contact with the web, so that such various problems are often encountered as paper powdering or scuffing in case of paper webs, etc., powder falling due to peeling and falling of coating composition in case of coated paper sheets, etc., scratches on the web surface in case of films, etc. Even simple contact with the shielding member has brought about such a problem as wear of the shielding member at an increasing coating speed and has been found not suitable for production-level operation for a long time. Multifunctional paper sheets often undergo multi-layer coating and a contact type-modified wet-on-wet coating method suffers from scraping of the coating solutions applied at the upstream positions also together with the entrained air and thus is not practical.
As mentioned above, a contact type method has been found highly effective as a method for shielding the curtain film from the air entrained around the moving web in a curtain coating apparatus for applying a coating solution to a continuously moving web, but it has been difficult to stably produce coated products without causing various problems such as paper powdering, scuffing, powder falling, scratching of web surfaces, wearing of shielding members, etc.
An object of the present invention is to provide a curtain coating apparatus and a curtain coating process capable of producing coated products of a good quality by eliminating an entrained air around a web at high speed coating, thereby stabilizing a curtain film.
As a result of extensive studies of the aforementioned problems, the present inventors have succeeded in establishing novel and efficient curtain coating apparatus and curtain coating process of the present invention.
A first aspect of the present invention provides a curtain coating apparatus for applying a coating solution to a continuously moving web, which comprises an air jet injector for injecting air in the upstream direction of a web, provided at a position upstream of a contact point of a curtain film with the web and a shielding member for shutting off entrained air generated by movement of the web, provided between the contact point and the air jet injector, thereby eliminating an influence of turbulent air flow and preventing the curtain film from suction toward the air jet injector.
A second aspect of the present invention provides a curtain coating apparatus for applying a coating solution to a continuously moving web, which comprises an air jet injector for injecting air in the upstream direction of a web, provided at a position upstream of a contact point of a curtain film with the web, a shielding member for shutting off entrained air generated by movement of the web, provided between the contact point and the air jet injector, thereby eliminating an influence of turbulent air flow and preventing the curtain film from suction toward the air jet injector, and a control member connected to the shielding member, the control member being provided at a position near and in parallel to the web, thereby preventing the entrained air from regrowth in the course between the air jet injector and the contact point.
A third aspect of the present invention provides a curtain coating apparatus for applying a coating solution to a continuously moving web, which comprises an air jet injector for injecting air in the upstream directions of a web, provided at a position upstream of a contact point of a curtain film with the web, a shielding member for shutting off entrained air generated by movement of the web, provided between the contact point and the air jet injector, a control member and a partition wall member, both of which are integrated with the shielding member to form a closed space, the air jet injector and an air suction means being provided in the closed space, thereby making the closed space a subatmospheric pressure zone.
A fourth aspect of the present invention provides a curtain coating apparatus for applying a coating solution to a continuously moving web, which comprises an air jet injector for injecting air in the upstream direction of a web, provided at a position upstream of a contact point of a curtain film with the web, the air jet injector being provided with an air injection pipe with air injection nozzles for shutting off entrained air generated by movement of the web, thereby eliminating an influence of tabulent air flow and preventing the curtain film from suction toward the air jet injector, and a partition wall member covering a distance upstream of a coater head, which extends from a position at the top of the air injection pipe with the air injection nozzles to a position near the web moving direction-shifting sector of a roll from the vertical direction to the horizontal direction, thereby making the partition wall member-enclosed zone a closed space, the air jet injector and an air suction means being provided in the closed space, thereby making the closed space a subatmospheric pressure zone.
In the third or fourth aspect of the present invention, it is preferable that the air jet injector is connected to the air suction means to send the air sucked up by the air suction means to the air jet injector as an air feed source.
In the first to fourth aspects of the present invention, the air jet injector of the curtain coating apparatus for applying a coating solution to a continuously moving web comprises an air injection pipe having a length equal to or more than the width of the web, and one end or both ends of the air injection pipe is connected to a feed pipe of an air feed pump, and the air injection pipe is provided with air injection nozzles for injecting the air at predetermined distances in the lateral direction of the web.
In the foregoing first to fourth aspects of the present invention, the air injection nozzles of the air injection pipe are provided at distances of 5-20 mm in the lateral direction of the web.
It is preferable that the air injection nozzles have a nozzle diameter of 1-3 mm. Furthermore, it is preferable that the air injection nozzles are in a circular or elliptical shape.
In the curtain coating apparatus according to the foregoing first to fourth aspects of the present invention, steam or mists is injected from the air jet injector in place of air.
A curtain coating process using a curtain coating apparatus for applying a coating solution to a continuously moving web according to the first to fourth aspects of the present invention, which comprises applying a coating solution to a web by an air jet injector provided at a position upstream of a contact point of a curtain film with the web and in an upstream direction, while maintaining a distance between the bottom side of an air injection pipe provided at the air jet injector and the continuously moving web at not more than 6 mm.
In the foregoing curtain coating process using a curtain coating apparatus for applying a coating solution to a continuously moving web according to the first to fourth aspects of the present invention one of air, steam and mists is injected from air injection nozzles provided in an air injection pipe of the air jet injector provided at a position upstream of a contact point of a curtain film with the web and in an upstream direction at a nozzle angle of 30xc2x0-60xc2x0 to the web.